This invention relates to a proportional-integral circuit widely used as a controller incorporated in various control devices. More particularly, this invention relates to a proportional-integral circuit which is composed of a proportional element for obtaining a signal V.sub.P proportional to an input signal V.sub.i, an integrator for obtaining an integrated signal V.sub.I by integrating the input signal V.sub.i, and an adder for obtaining the sum of the output V.sub.P of the proportional element and the output V.sub.I of the integrator. More specifically, this invention relates to a proportional-integral circuit of the kind above described which is used for automatically controlling a controlled system according to the output of the proportional-integral circuit (hereinafter referred to as an automatic mode) and for manually controlling the controlled system according to the output of a manual handling source provided at the output end of the adder (hereinafter referred to as a handling mode), and in which means for changing over between the automatic mode and the handling mode is provided so that the handling mode can be bumplessly changed over to the automatic mode without imparting any shock to the controlled system.
As described above, a proportional-integral circuit is composed of a proportional element for obtaining a signal V.sub.P proportional to an input signal V.sub.i, an integrator for obtaining an integrated signal V.sub.I by integrating the input signal V.sub.i, and an adder for obtaining the sum V.sub.PI of V.sub.P and V.sub.I. A manual handling source producing an output V.sub.H is provided at the output end of the adder. In the automatic mode, a controlled system is controlled according to the output V.sub.PI of the adder, and the output V.sub.H of the manual handling source follows up the output V.sub.PI of the adder.
When the automatic mode is changed over to the handling mode, the output V.sub.H of the manual handling source is applied to the controlled system in lieu of the adder output V.sub.PI. Since V.sub.PI = V.sub.H in the automatic mode, changeover from the automatic mode to the handling mode can be attained in bumpless fashion. Considering subsequent change-over from the handling mode to the automatic mode again, it is necessary that the adder output V.sub.PI should follow up the output V.sub.H of the manual handling source in the handling mode.
In order to ensure bumpless change-over from the handling mode to the automatic mode in a state as above described, the proportional-integral circuit is arranged to operate in a manner as described below in the handling mode. In the handling mode, an input signal V.sub.i is applied to the proportional element to obtain a signal V.sub.P proportional to the input signal V.sub.i. A resistor is connected in parallel with an integrating capacitor in the integrator so that the integrator possesses an adding function. The input signal V.sub.i is not applied to the integrator, and the ouput V.sub.p of the proportional element and the output V.sub.H of the manual handling source are applied to the integrator to obtain the sum of V.sub.P and V.sub.H. The adder computes the sum V.sub.PI of the output V.sub.P of the proportional element and the output V.sub.I of the integrator. Since, in this case, the integrator output V.sub.I consists of the proportional component V.sub.P and the output V.sub.H of the manual handling source, the output V.sub. PI of the adder can follow up the output V.sub.H of the manual handling source when V.sub.P included in the integrator output V.sub.I and V.sub.P obtained by the proportional element cancel each other.
This is carried out, for example, in a manner as described below. The proportional element is composed of an input resistor of resistance value R.sub.1 through which V.sub.i is applied, a first operational amplifier A1, and a feedback resistor of resistance value R.sub.2 connected in parallel with the operational amplifier A1. The integrator is composed of an input resistor of resistance value R.sub.3 through which V.sub.i is applied, a second operational amplifier A2, and an integrating capacitor of capacitance value C connected in parallel with the operational amplifier A2. The adder is composed of an input resistor of resistance value R.sub.7 through which proportional element output V.sub.P is applied, another input resistor of resistance value R.sub.8 through which integrator output V.sub.I is applied, a third operational amplifier A3, and a feedback resistor of resistance value R.sub.9 connected in parallel with the operational amplifier A3. In the handling mode, a feedback resistor of resistance value R.sub.5 is connected in parallel with the second operational amplifier A2 to provide the adding function to the integrator. It is supposed that an input resistor for applying V.sub.P to the integrator in the handling mode has a resistance value R.sub.4, and an input resistor for applying V.sub.H to the integrator in the handling mode has a resistance value R.sub.6.
The relations among the resistance values of the resistors above described are as follows: EQU R.sub.4 = R.sub.5 = R.sub.6 ( 1) EQU R.sub.7 = R.sub.8 = R.sub.9 ( 2) ##EQU1## K.sub.P in the equation (3) is the proportional gain of the output V.sub.P of the proportional element relative to the input signal V.sub.i. Due to the fact that the resistance values of the individual resistors are selected to provide the relations above described, the output V.sub.P of the proportional element is given by the equation (4), but the sign is inverted since the output appears from the operational amplifier. The output V.sub.I of the integrator and the ouput V.sub.PI of the adder in the handling mode are respectively given by the equations (5) and (6). ##EQU2##
The equation (5) represents the output of the integrator, and the resistor R.sub.5 is connected in parallel with the capacitor C in order that the integrator can possess the adding function in the handling mode. This circuit structure of the integrator in the handling mode is commonly called a first order lag circuit. Therefore, this circuit has a time constant T (T = CR.sub.5) determined by C and R.sub.5. The equation (5) indicates the output of the integrator in the state in which the circuit is stabilized with lapse of time greater than the time constant T.
It is seen from the above description that change-over from the handling mode to the automatic mode can be performed in bumpless fashion when the resistance values of the resistors associated with the operational amplifiers are selected to satisfy the equations (1) and (2).
In the above description, however, saturation of the operational amplifiers is not taken into account, and the relation given by the equation (6) does not hold when the operational amplifiers are saturated. This point will be discussed while taking actual numerical values. The values of V.sub.i and V.sub.H are generally .+-.10 volts, and the saturation voltage of the operational amplifiers in this case is generally .+-.12 volts. Therefore, in the case of the integrator which is most easily saturated, the relation between V.sub.H and V.sub.i must satisfy the inequality (7). EQU -12 (v) &lt; K.sub.P V.sub.i - V.sub.H &lt; + 12 (v) (7)
In the inequality (7), V.sub.i and V.sub.H take any suitable value within the range of +12 volts and -12 volts. Suppose that V.sub.H is -8 volts, then the integrator saturates when K.sub.P V.sub.i is greater than 4 volts. In such a case, V.sub.i which provides the output of the proportional element is not equal to V.sub.i included in the output V.sub.I of the integrator, and the output V.sub.PI of the adder is not equal to the output V.sub.H of the manual handling source. When the handling mode is changed over to the automatic mode under such a condition, a shock may be imparted to the controlled system due to the difference between V.sub.H and V.sub.PI, and bumpless change-over from the handling mode to the automatic mode may not be attained.
It will thus be seen that obstruction against bumpless change-over from the handling mode to the automatic mode due to saturation of the operational amplifiers in the proportional-integral circuit is extremely inconvient and undesirable from the standpoint of handling. Further, this provides considerable restrictions on the application of the proportional-integral circuit to actual plants. Therefore, such restrictions must be eliminated by employing suitable means which satisfies both the condition required for fulfilling the function of the proportional-integral circuit and the condition required for the bumpless change-over.